1. Field of the Invention
The present invention relates to a surface acoustic wave resonator and surface acoustic wave filter for use in a portable phone.
2. Description of the Related Art
In a communication device such as a portable phone, a surface acoustic wave filter is used for extracting only a required frequency. Such a surface acoustic wave filter is required to have various characteristics corresponding to the communication device, such as a filter characteristic of low loss and broad band.
As a method of achieving the low loss and broad band filter characteristic, generally, a ladder-type surface acoustic wave filter where one-port resonators are disposed alternately in a series arm and a parallel arm is used. Such a surface acoustic wave filter generally has a structure shown in FIG. 19. In the surface acoustic wave filter shown in FIG. 19, two surface acoustic wave resonators 200 are connected to the series arm, and two surface acoustic wave resonators 300 are connected to the parallel arm, on the surface of piezoelectric substrate 100. Surface acoustic wave resonators 200 and 300 have a pair of inter-digital transducers (hereinafter referred to as “IDT”) including a plurality of electrode fingers and reflectors disposed on opposite sides of the transducer. Comb electrodes of IDTs constituting surface acoustic wave resonators 200 of the series arm are connected to input-output terminals 410 and 420, respectively.
The frequency of the communication device, such as the portable phone employing such a surface acoustic wave filter, has increased, and a surface acoustic wave filter responsive to the increase is required. For responding to the increase in frequency, technology of decreasing the propagation loss using a 38° - 46° Y cut X propagation lithium tantalate (LiTaO3) substrate as the piezoelectric substrate has been developed.
A CDMA (code division multiple access) system is employed as a modulation method in the portable phone, and suppression of a fine ripple in a pass band is required.
Japanese Patent Unexamined Publication No. 2001-119260 discloses a surface acoustic wave filter for suppressing a ripple. The surface acoustic wave filter has at least two surface acoustic wave resonators connected to a series arm and at least one surface acoustic wave resonator connected to a parallel arm, on the surface of the 38°-46° Y cut X propagation lithium tantalate (LiTaO3) substrate. The surface acoustic wave resonators of the series arm have an excitation electrode formed of a plurality of electrode fingers. When the smallest number of electrode finger pairs of the surface acoustic wave resonators of this series arm is assumed to be N (N is a natural number), the number of electrode finger pairs in at least one of the surface acoustic wave resonators of the other series arm is set to be n×N (n is a natural number), namely different from N.
Thanks to this structure, a ripple caused by the surface acoustic wave resonator of the series arm having the smallest number of electrode finger pairs or the smallest number of reflecting electrodes can be cancelled by the surface acoustic wave resonator of the series arm where the number of electrode finger pairs or the number of reflecting electrodes is not a positive integral multiple.
In the surface acoustic wave filter disclosed by the above-mentioned document, the ripple is reduced as a whole by displacing the positions of the ripples of a plurality of series resonators. However, a ripple exists in each resonance characteristic, so that reduction of the ripples is restricted in this structure.
In the conventional general structure shown in FIG. 19, when the number of electrode finger pairs in a series resonator is reduced, the ripple in a pass band disadvantageously becomes large.
The present invention addresses the conventional problems, and provides a surface acoustic wave resonator and surface acoustic wave filter having a small ripple.